Relays have long been used in both consumer and commercial appliances and machinery to provide automated or electrically controlled switching operation. One of the benefits of such relays is that they allow the use of “low level” signals to switch “high level” power. That is, a typical relay includes at least one coil that pulls in or controls the switching of the main relay contacts. For some types of magnetically held relays, de-energization of the relay coil will cause the main relay contacts to open under action of a spring force or other mechanical bias. Such held relays, therefore, require that the coil be energized during the period of main contact closure (or opening in a normally-closed relay configuration). Another type of single coil relay is known as a cutthroat relay. In this relay the state of the contacts is transitioned by momentarily energizing the relay coil. That is, to open the relay if the contacts are currently closed, the relay coil is pulsed. Within the relay, a cutthroat mechanism switches over so that upon subsequent energization of the relay coil the contacts will then re-close. Latching type relays utilize two separate coils, one dedicated to open the contacts, and one dedicated to close the contacts. That is, if the contacts are currently closed, the trip coil may be pulsed to cause the contacts to open. Once the contacts have opened, there is no need to maintain energization of the trip coil. To close the contacts from this state, the close coil is energized.
While these relays utilize an electronic control signal to control the position of the main relay contacts, the contacts themselves are mechanical structures. As such, they are bound by the laws of physics. Because of this, their physical properties must be taken into account in the control circuitry and control logic for the relays. As illustrated in FIG. 8, one of the physical properties that must be taken into account when utilizing relays is the time lag between the energization of the relay coil (depicted as line 800) and the actual transition of the relay contacts (as illustrated by the relay output voltage line 802). As may be seen from this FIG. 8, the relay control circuitry energizes the relay coil at time T0. Once energized, the relay coil establishes a magnetic flux that will, in this example, close the relay contacts. The actual contact closure takes place at time T1. As indicated by line 802, however, the initial closing at time T1 is typically followed by a short period of relay contact bounce before the relay contacts maintain their closed state at time T2. This mechanical bounce is a result of the kinetic energy that is generated as the relay contacts are accelerated toward one another under the influence of the magnetic flux generated by the relay coil.
A different, but somewhat related phenomenon of intermittent contact bounce occurs between the relay contacts when they are opened. During the trip operation of an electrically held relay, the relay coil is de-energized and the relay contacts are allowed to be opened by a mechanical bias force, often provided by a spring. However, the flux generated by the relay coil is not extinguished immediately. As such, there is some initial contention between these two opposing forces. Additionally, the current flow through the relay contacts also plays a part in the slight bounce or chatter during the trip operation. With current flowing through the relay contacts, initial separation of the contacts results in an arc being drawn between the two contacts which tends to pull the contacts together. Until the spring force can overcome these opposing forces, inconsistent opening may occur for a short time. Similar bounce or chatter is also seen for the other types of relays described above that require coil energization to open the contacts.
While the delay in opening and closing the relay contacts can be compensated in the control circuitry and logic, the contact bounce phenomenon occasionally results in a mechanical failure of the relay. Specifically, and especially when supplying high in-rush capacitive, motor, lamp, and overloads through the relay, the relay bounce results in an arc being drawn between the relay contacts at each bounce. As a result of this arcing, the metal that forms the relay contacts may become molten at a small and localized point. When the contacts come back together, this molten material of the relay contacts may form a small tack weld. This tack weld prevents the relay contacts from opening under normal operation. A similar situation may occur during the opening of the relay coil, especially with relays that utilize separate trip coils due to the time required to establish sufficient flux to separate the contacts in high current applications. This problem may become especially acute in applications that use coil suppression techniques in the driver circuitry of such trip coils.
As a result of the relay tack weld failure, the relay contacts remain closed, and the load to which they are connected cannot be de-energized. If this problem happens to the control relay of, for example, a compressor in a refrigerator, the compressor cannot be de-energized once the temperature in the freezer or fresh food compartment has reached its desired set point. This will result in the temperature set point being exceeded by continued operation of the compressor. As a result, the owner will be forced to make a service call to correct this problem.
Because the actual area of the relay contact surface that is tack welded is typically very small, the removal of the relay by service personnel to investigate the cause of the failure often results in breaking this physical tack weld. When the relay is subsequently tested, it may operate normally. This may be reported as a “could-not-duplicate” failure or may result in further, needless investigation of other potential causes for failure. Often, this may lead to a costly replacement of the control board that contains the relay driver circuitry. This may well result in needless loss of time and additional expense for the consumers, not to mention the frustration that may be caused by the initial failure of the relay itself.
There exists, therefore, a need in the art for a relay control method that can detect a relay tack weld failure, and attempt to correct this failure before service personnel needs to be called.